Biomedical Engineering Reference
In-Depth Information
The total reaction can be expressed as follows:
COD
+
+
Cholesterol
O
2
cholest-4-en-3-one
H
2
O
2
(14.12)
Based on Reaction 14.12, COD can also be used to prepare amperometric biosensor, poten-
tiometric biosensor, and optical biosensor for cholesterol detection. Furthermore, biosensors con-
structed with the combination of COD and CEH can be used for the detection of cholesteryl ester.
Examples of biosensors prepared by COD or by the combination of CEH and COD are listed in
Table 14.4.
Biosensors composed of COD or CEH/COD can be used to detect the contents of cholesterol in
serum and sterol medicines. In addition, they are involved in the tracking measurement of Zn
2
+
and
other heavy metal ions in water with high facility and effi ciency [44].
14.1.1.3
Acetylcholinesterase (AChE)/Choline Oxidase
AChE ( E.C. 3.1.1.7 ) is an important component of cholinergic synapses in the peripheral and central
nervous systems, which are partly responsible for terminating the actions of the neurotransmitter
acetylcholine in all the vertebrates and invertebrates [45].
AChE splits into four subunits in the presence of guanidine and mercaptoethanol, and each
subunit has one-fourth of the molecular weight of the original enzyme. Examination of the
C-terminal residues by two independent methods, namely hydrazinolysis and enzymatic hydrolysis
by carboxypeptidase A, revealed that there were two types of polypeptide chains in AChE. This
examination suggests that AChE has a dimeric hybrid structure, with two α and two β chains [46].
AChE can catalyze the hydrolysis of acetylcholine to produce choline and acetic acid specifi -
cally as in Reaction 14.13:
AC
h
E
Acetylcholine
+
H
2
O
choline
+
CH
3
COOH
(14.13)
Choline oxidase (ChOD, E.C. 1.1.3.17) can be extracted from
Cylindrocarpon didymium
M-1
[47],
Alcaligenes
sp. [48], and
Arthrobacter globiformis
[49]. Based on amino acid sequence com-
parisons, the enzyme ChOD can be grouped in the glucose-methanol-choline (GMC) oxidoreduc-
tase enzyme superfamily [50], which utilizes FAD as cofactor for catalysis and uses nonactivated
primary alcohols as substrate.
ChOD catalyzes the four-electron oxidation of choline to glycine betaine (
N,N,N
- trimethylglycine,
betaine), while molecular oxygen acts as the primary electron acceptor in Reaction 14.14 [49].
ChOD
Choline
+
2O
2
+
H
2
O
glycine betaine
+
H
2
O
2
(14.14)
ChOD is an important enzyme in Reaction 14.15 because glycine betaine is one of the limited
compatible solutes that accumulate to high levels in the cytoplasm of cells to prevent dehydration
and plasmolysis in adverse hyperosmotic environments [51].
Especially, organophosphorus (OP) pesticide is similar to acetylcholine in molecular morphol-
ogy, which can combine the active ester locus of AChE and then restrain the activity of the enzyme.
There is a fi ne linear relationship between the concentration of OP pesticide and the inhibition
degree of AChE, while the activity of AChE decides the outcome of choline and acetic acid. Based
on this character, potentiometric and optical biosensors can be prepared for the detection of OP
pesticide by measuring the decrease of H
+
concentration. When ChOD is coworked with AChE,
the Reaction 14.14 will be weakened by OP pesticide, so amperometric biosensor for OP pesticide
detection can also be constructed on the basis of monitoring the decrease of hydrogen peroxide.
Biosensors composed of AChE or AChE/ChOD for OP pesticide detection are listed in Table 14.5.
